(1) Field of the Invention
The invention relates to an echo canceler particularly for use in a data modem for simultaneous two-way transmission of data signals over two-wire connections.
(2) Description of the Prior Art
Usually, modern transmission systems consist of a combination of two-wire and four-wire connections. Four-wire connections consist of two one-way paths, namely a one-way transmit path and a one-way receive path. A two-wire connection is constituted by a two-way path over which signals can be simultaneously transmitted into two directions in one and the same frequency band. These different paths are interconnected by means of a hybrid.
As known, a hybrid is constituted by a four-port circuit. A first port, the so-called transmit-port, is connected to the one-way transmit path, a second port, the so-called receive-port, is connected to the one-way receive path, a third port, the so-called cable port is connected to the two-way path and a balancing network is connected to the fourth port, the so-called balancing port. This balancing network is intended to match the hybrid to the cable impedance. When this balancing network is perfectly adjusted, a signal in the transmit path will appear only at the cable port. If, on the contrary, a signal is applied to the cable port over the two-way path, this signal will only appear in the receive path.
As each two-way path has one or more discontinuities from which a signal can be reflected, a reflected version of the signal occurring in the transmit path may be expected in the receive path.
Owing to the diverse cable lengths and cable types, the cable impedance is usually not exactly known, so that a perfect adjustment of the balancing network is impossible. Consequently, a portion of the signal occurring in the transmit path will directly enter the receive path via the hybrid.
Those portions of the signal present in the transmit path and occurring in the receive path will, as customary, be denoted as echo signal.
Such echo signals have a particularly negative influence on the quality of the signal in the receive path. In order to improve this quality, circuits have been designed to cancel the echo signals or at least to reduce the influence to a considerable extent. Such circuits are known as echo cancelers.
Reference 1 of paragraph D shows that an echo canceler usually comprises:
an adjustable signal processing arrangement coupled to the one-way transmit path; PA1 a difference producer connected to the one-way receive path for generating a residual signal which indicates the difference between the signal occurring in the receive path and the signal produced by the signal processing arrangement; PA1 an adjusting device responsive to control signals for adjusting the signal processing arrangement; PA1 a control signal generator for generating the above-mentioned control signals and being responsive to the above-mentioned residual signal. PA1 a generation device for generating an auxiliary signal which is not correlated to the residual signal; PA1 an adding device for adding the auxiliary signal and the residual signal together for generating a limiter input signal which is applied to the limiter circuit.
Starting from a signal occurring in the transmit path, the signal processing arrangement generates a synthetic echo signal, the shape of which corresponds as closely as possible with the echo signal to be expected. The degree of similarity between the synthetic echo signal and the real echo signal is determined by the setting of the signal processing arrangement which is catered for by the adjusting device to which the output signal of the control signal generator is applied. This adjusting device and this control signal generator are usually constructed so that they are capable of deriving from the residual signal a signal which is a measure of the nonsuppressed echo signal present in the residual signal, the so-called residual echo, and of so adjusting the signal processing arrangement in an iterative manner by means of this signal that a minimum value is obtained of the mean square value of the residual echo.
Reference 1 proposes in particular to provide the control signal generator with a limiting circuit to which the residual signal is applied, this limiting circuit converting this signal into a sequence of positive and negative pulses, which are indicative of the polarity of this residual signal. The pulse train thus obtained is now applied to the adjusting device by way of control signal.
Such a construction of a control signal generator is particularly useful when the echo canceller must be implemented by means of digital modules only, (see for example, references 2 and 3). The limiting circuit constitutes, namely, a particularly simple analog-to-digital converter for the analog residual signal.
However, Applicants have now ascertained that, although the limiting circuit can be used very successfully to digitize the residual signal in a digital echo canceller used in a so-called plesiochronous transmission system, such a digitization in an echo canceller intended to be used in a so-called homochronous transmission system results in an insufficient compensation of the echo signal. In such circumstances a limiting circuit must be replaced by an analog-to-digital converter, which converts the residual signal into a sequence of code words each comprising a greater number of bits. However, such an analog-to-digital converter is relatively costly, it dissipates much energy and is an element which is difficult to integrate.